Not applicable.
This invention relates to electronic imaging apparatus and techniques and more particularly to an electronic imager capable of performing selectable signal processing functions on the output signals of detector elements with fixed digital logic circuitry.
Modern imaging has its roots in the nineteenth century with the advent of film which is used today for diverse applications, including generating common photographs and radiographic medical images using x-rays. Dunng the past ten to twenty years, electronic imaging has become common in many fields and has totally replaced film systems in some applications. The term xe2x80x9celectronic imagingxe2x80x9d as used herein includes electro-optical imaging in the infrared, visible, and ultraviolet regions of the spectrum, and also in the higher energy regions of the spectrum including soft and hard x-rays and gamma-rays.
In its simplest form, electronic imaging is performed by intercepting radiation in the form of photons from an object of interest or scene to be viewed. The photons may be generated by various sources, such as astronomical sources including the sun and stars, other sources of soft x-rays (photons with energy below 10 kev), x-ray tubes and other sources of hard x-rays (photons with energy equal to or above 10 kev), and gamma ray isotopes or other high energy sources of photons above 50 kev. The photons incident on the object to be viewed can be provided directly from an energy source or can be reflected by one or more objects.
Prior to interception, the photons can travel or transit through the earth""s atmosphere, the near vacuum of outer space, water, tissue or organs or other elements of a patient in medical applications, other objects to be imaged and examined, or any other medium which may or may not degrade the image or provide information of interest. The photons may pass through lenses, be reflected by mirrors, or be affected by baffles or other components. The intercepted photons may be from one band of the electromagnetic spectrum, from more than one band (i.e., multi-spectral), from many bands (i.e., hyperspectral), or from all bands.
In electronic imaging, the interception of photons is accomplished by imaging, or detector arrays. Detector arrays typically are divided into detector, or picture elements (i.e., pixels) and include a plurality of pixels arranged in a linear array or two-dimensional array. The intercepted photons cause electrical signals in various analog forms, such as a voltage, current, or charge, to be generated by the detector elements. Commonly available detector array configurations for electronic imaging include point scan, slit scan, slot scan (sometimes referred to as xe2x80x9cpush broomxe2x80x9d) and fixed two-dimensional image receptors. Such detector arrays are commonly located in vehicles including aircraft and spacecraft, medical facilities, airports, industrial facilities, homes, offices, and a variety of other locations and can be subsystems of cameras or other equipment.
In many applications, the detector array is enabled to intercept photons for an interval of time (i.e., an imaging interval) and after that interval, the resulting electrical signal generated in each pixel is read out in some fashion and presented to a user or operator of the imager and/or is stored in a memory device for further image processing. It is sometimes necessary or desirable to measure pixel output signals many times during a single imaging interval and compute a function of the measured values. For example, in an x-ray detection system described in U.S. Pat. No. 5,665,969 entitled X-RAY DETECTOR AND METHOD FOR MEASURING ENERGY OF INDIVIDUAL X-RAY PHOTONS FOR IMPROVED IMAGING OF SUBJECTS USING REDUCED DOSE, a weighted sum function of many single photon measurements is computed during each imaging interval in order to improve the resulting image. The patent describes the use of a transistor switch for performing the weighted sum function in which the weighting given to a particular photon measurement is determined by resistance values of resistors and the transistor switch.
According to the invention, an electronic imager comprises a detector array including a plurality of radiation sensitive elements, a plurality of analog-to-digital conversion circuits, each responsive to an input signal from at least one of the radiation sensitive elements and to at least one threshold signal for converting the input signal into a digital signal, and a digital logic circuit. The digital logic circuit is responsive to the digital signal from at least one of the analog-to-digital conversion circuits and provides an output signal related to the input signal by a selectable function, wherein the function is selected by adjusting the threshold signal.
With this arrangement, different signal processing functions can be performed with common imager apparatus or a common imager design, thereby increasing the utility of the imager apparatus or design. As will become apparent, different signal processing functions include different function types (e.g., an exponential weighting function or an arbitrary function) and/or different function parameters (e.g., a different set of weights for a weighting function). For example, in a medical application, different signal processing functions may be used to enhance the imaging in different tissue types or as a function of different tissue characteristics such as tissue density and tissue thickness. Also, the imager apparatus or design may be used in different applications, such as medical and environmental monitoring applications, by varying the signal processing functions in a variable apparatus or a fixed, specialized apparatus which could be member of a family of apparatus which are variants of a common design. Further, this versatility is achieved in a manner which permits the use of digital logic circuitry, which is often preferable to analog circuitry for reasons of size, cost and simplicity.
The digital logic circuit is fixed (i.e., non-adjustable). The use of fixed digital logic circuitry is possible since it is the threshold signal provided to the analog-to-digital conversion circuit which is adjusted to select a particular signal processing function. This threshold signal adjustment may be made on a variable apparatus prior to or during use or during the manufacture or factory preadjustment of a specialized apparatus. Thus, the versatility of the imager apparatus or family of apparatus is achieved without the added cost of providing additional or variable digital logic circuits to implement different signal processing functions.
Also described is a method for processing an input signal from a radiation sensitive element with an electronic imager including the steps of converting the input signal into a digital signal by comparing the input signal to at least one threshold signal, processing the digital signal to provide an output signal having a relationship with respect to the input signal determined by the threshold signal, and adjusting the threshold signal in order to change the relationship between the output signal and the input signal. By adjusting the threshold signal, the above-described method can be used in various imaging applications.
An analog-to-digital conversion circuit according to the invention includes a comparator circuit responsive to an analog input signal and to a plurality of threshold signal levels for comparing the analog input signal to the threshold signal levels, and a control circuit for generating the threshold signal levels. At least one increment between adjacent threshold signal levels is not equal to increments between other adjacent threshold signal levels. In one embodiment, the comparator circuit includes a plurality of comparators, each receiving a respective threshold signal level substantially simultaneously. In an alternative embodiment, the plurality of threshold signal levels are provided sequentially to the comparator circuit.
The use of such an analog-to-digital conversion circuit in an electronic imager permits various signal processing functions to be performed on the pixel signals simply by adjusting the threshold signal levels and concomitantly, the increments between adjacent threshold signal levels according to the desired function. The described analog-to-digital conversion circuit can be contrasted to conventional analog-to-digital converters in which the increments between adjacent threshold levels are equal in order to provide a linear relationship between the analog input signal and the digital output signal.